A piston ring sliding inside an engine slides on an inner circumferential surface of a sleeve (cylinder) in accordance with reciprocating motion of a piston which is mounted in a ring groove formed on an outer circumferential surface of the piston. Accordingly, low-friction properties and abrasion resistance are particularly required among sliding characteristics. Therefore, in the related art, an amorphous carbon film (which will hereinafter also be referred to simply as a “carbon film”) is formed on a surface of the piston ring such that sliding characteristics are ensured.
In this case, in order to sufficiently exhibit low-friction properties or abrasion resistance described above, there is a need to increase the thickness of a carbon film to a certain degree. However, since amorphous carbon films have strong internal stress, it has not been easy to form one layer of a carbon film to be thick.
Therefore, for example, Patent Document 1 proposes that internal stress is relaxed by layering two kinds of carbon layers that differ in hardness (hard layer and soft layer) in two or more layers and forming a carbon film, and the carbon film is formed to be a thick film having a film thickness of 5.0 μm or greater.
In this case, the performance (low-friction properties and abrasion resistance) of the film in its entirety is improved by appropriately regulating mechanical properties of each of the hard layer and the soft layer and thickening the carbon film. However, due to the different mechanical properties between the hard layer and the soft layer, there is concern that discontinuity of mechanical properties will be caused in an interface of layers and separation will occur between the layers.
In regard to the separation between layers, Patent Document 2 proposes a technology in which interlayer adhesion is improved and occurrence of separation is suppressed. In Patent Document 2, two kinds of layers having compositions different from each other are repetitively formed, and an inclined layer in which the composition continuously changes is formed between the layers, so that adhesion between two kinds of layers is ensured.